Axel Lorke

Optical emission from a charge-tunable quantum ring

Quantum dots or rings are artificial nanometre-sized clusters that confine electrons in all three directions. They can be fabricated in a semiconductor system by embedding an island of low-bandgap material in a sea of material with a higher bandgap. Quantum dots are often referred to as artificial atoms because, when filled sequentially with electrons, the charging energies are pronounced for particular electron numbers; this is analogous to Hunds rules in atomic physics. But semiconductors also have a valence band with strong optical transitions to the conduction band. These transitions are the basis for the application of quantum dots as laser emitters, storage devices and fluorescence markers. Here we report how the optical emission (photoluminescence) of a single quantum ring changes as electrons are added one-by-one. We find that the emission energy changes abruptly whenever an electron is added to the artificial atom, and that the sizes of the jumps reveal a shell structure.

Spectroscopy of nanoscopic semiconductor rings

Making use of self-assembly techniques, we realize nanoscopic semiconductor quantum rings in which the electronic states are in the true quantum limit. We employ two complementary spectroscopic techniques to investigate both the ground states and the excitations of these rings. Applying a magnetic field perpendicular to the plane of the rings, we find that, when approximately one flux quantum threads the interior of each ring, a change in the ground state from angular momentum l = 0 to l = -1 takes place. This ground state transition is revealed both by a drastic modification of the excitation spectrum and by a change in the magnetic-field dispersion of the single-electron charging energy.

Intermixing and shape changes during the formation of InAs self-assembled quantum dots

The initial stages of GaAs overgrowth over self-assembled coherently strained InAs quantum dots (QDs) are studied. For small GaAs coverages (below 5 nm), atomic force microscopy (AFM) images show partially covered island structures with a regular size distribution which are elongated in the [011] direction. Analysis of the AFM profiles show that a large anisotropic redistribution of the island material is taking place during the initial GaAs overgrowth. Short time annealing experiments together with photoluminescence spectroscopy on annealed QDs are consistent with a Ga and In intermixing during the overgrowth. Surface QDs capped with 5 nm or more GaAs show a strong luminescence intensity indicating that surface QDs are remarkably insensitive to surface recombination effects.

Epitaxially Self-Assembled Quantum Dots

Nanometer-scale islands that form spontaneously on a semiconductor substrate have atomlike properties and potential applications in optical and optoelectronic devices, quantum computing, and information storage.

Shell structure and electron-electron interaction in self-assembled InAs quantum dots

Using far-infrared spectroscopy, we investigate the excitations of self-organized InAs quantum dots as a function of the electron number per dot, 1 ≤ ne ≤ 6, which is monitored in situ by capacitance spectroscopy. Whereas the well-known two-mode spectrum is observed when the lowest (s-) states are filled, we find a rich excitation spectrum for ne ≥ 3, which reflects the importance of electron-electron interaction in the present, strongly non-parabolic confining potential. From capacitance spectroscopy we find that the electronic shell structure in our dots gives rise to a distinct pattern in the charging energies which strongly deviates from the monotonic behavior of the Coulomb blockade found in mesoscopic or metallic structures.

Silicon nanoparticles : Absorption, emission, and the nature of the electronic bandgap

Silicon nanoparticles synthesized in the gas phase are studied. From time-resolved photoluminescence measurements we determine, quantitatively, the size-dependence of the oscillator strength of the nanoparticles. We investigate experimentally the absorption and photoluminescence emission of nanoparticle ensembles with a broad size distribution. Using a model which accounts for size-effects in both oscillator strength and quantum-confinement, we are able to calculate absorption and emission spectra of ensemble samples. From these results we have determined, whether silicon nanoparticles should be regarded as indirect or direct semiconductors. Moreover, we systematically study the influence of the particle size-distribution on the optical spectra.

Single-chip fused hybrids for acousto-electric and acousto-optic applications

The combination of the electronic and optical properties of a semiconductor hetero-junction and the acoustic properties of a piezoelectric substrate material yields a new class of very promising hybrids for potential acousto-electric and acousto-optic applications. LiNbO3/GaAs hybrids have been fabricated using the epitaxial lift-off technique resulting in unusually large acousto-electric and acousto-optic interaction between the quasi two-dimensional electron system in the semiconductor and surface acoustic waves on the piezoelectric substrate. Field effect tunability of the interaction at room temperature is demonstrated and possible device applications are discussed. Photoluminescence measurements show the influence of the acousto-electric fields on the optical properties of quantum well structures.

The dynamics of tunneling into self-assembled InAs dots

Using frequency-dependent capacitance spectroscopy, the dynamics of tunneling into arrays of self-assembled InAs quantum dots is investigated with respect to sample geometry, Coulomb interaction, and magnetic field. An equivalent resistance-capacitance circuit is derived which allows us to determine the tunneling times for each state of the dots. The different tunneling times for different many-particle states are explained by a reduced tunneling barrier and Coulomb interaction. A magnetic field applied perpendicular to the tunneling direction results in a strong suppression of the charging signal, which is attributed to enhanced localization caused by the magnetic field. Calculations for three-dimensional to zero-dimensional magnetotunneling can account for the experimental data.

Growth and Electronic Properties of Self-Organized Quantum Rings

A method is described which can be used to grow self-organized, nanoscopic InGaAs ring structures on GaAs substrate. Starting from self-organized InAs dots, the crucial step for the ring formation is a short annealing phase after the dots have been covered by a thin GaAs layer. Spectroscopic data are reviewed which show that the ring morphology can be preserved even after the InGaAs islands have been covered by additional cladding layers for the realization of electronically or optically active devices.

Excitons in self-assembled quantum ring-like structures

Abstract A remarkable morphological change of self-assembled InAs quantum dots takes place during growth if a pause is introduced after overgrowing the dots with a few nm of GaAs. Atomic force microscopy indicates that the shape of the dots changes lens-like to ring-like. We report here the results of capacitance and interband transmission experiments on such ring-like structures embedded in a GaAs matrix. In particular, we compare the electronic properties of conventional dots with those of the rings. Significant changes are found which qualitatively support a quantum ring model.